The highly reduced genome of an enslaved algal nucleus

Susan Douglas, Stefan Zauner, Martin Fraunholz, Margaret Beaton, Susanne Penny, Lang-Tuo Deng, Xiaonan Wu, Michael Reith, Thomas Cavalier-Smith () and Uwe-G Maier
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Susan Douglas: National Research Council of Canada Institute for Marine Biosciences and Program in Evolutionary Biology, Canadian Institute of Advanced Research
Stefan Zauner: Cell Biology and Applied Botany, Philipps-University Marburg
Martin Fraunholz: Cell Biology and Applied Botany, Philipps-University Marburg
Margaret Beaton: Program in Evolutionary Biology, Canadian Institute of Advanced Research, University of British Columbia
Susanne Penny: National Research Council of Canada Institute for Marine Biosciences and Program in Evolutionary Biology, Canadian Institute of Advanced Research
Lang-Tuo Deng: Program in Evolutionary Biology, Canadian Institute of Advanced Research, University of British Columbia
Xiaonan Wu: Program in Evolutionary Biology, Canadian Institute of Advanced Research, University of British Columbia
Michael Reith: National Research Council of Canada Institute for Marine Biosciences and Program in Evolutionary Biology, Canadian Institute of Advanced Research
Thomas Cavalier-Smith: Program in Evolutionary Biology, Canadian Institute of Advanced Research, University of British Columbia
Uwe-G Maier: Cell Biology and Applied Botany, Philipps-University Marburg

Nature, 2001, vol. 410, issue 6832, 1091-1096

Abstract: Abstract Chromophyte algae differ fundamentally from plants in possessing chloroplasts that contain chlorophyll c and that have a more complex bounding-membrane topology1. Although chromophytes are known to be evolutionary chimaeras of a red alga and a non-photosynthetic host1, which gave rise to their exceptional membrane complexity, their cell biology is poorly understood. Cryptomonads are the only chromophytes that still retain the enslaved red algal nucleus as a minute nucleomorph2,3,4. Here we report complete sequences for all three nucleomorph chromosomes from the cryptomonad Guillardia theta. This tiny 551-kilobase eukaryotic genome is the most gene-dense known, with only 17 diminutive spliceosomal introns and 44 overlapping genes. Marked evolutionary compaction hundreds of millions of years ago1,4,5 eliminated nearly all the nucleomorph genes for metabolic functions, but left 30 for chloroplast-located proteins. To allow expression of these proteins, nucleomorphs retain hundreds of genetic-housekeeping genes5. Nucleomorph DNA replication and periplastid protein synthesis require the import of many nuclear gene products across endoplasmic reticulum and periplastid membranes. The chromosomes have centromeres, but possibly only one loop domain, offering a means for studying eukaryotic chromosome replication, segregation and evolution.

Date: 2001
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DOI: 10.1038/35074092

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